1. Field of the Invention
The present invention generally relates to digital systems, and more particularly to the galvanic isolated digital output circuit of digital systems.
2. The Prior Arts
An external load or system under the control of a digital system is usually operated under a high voltage while the controlling digital system is usually operated under a low voltage. The digital output circuit of the digital system, therefore, usually employs a galvanic isolation device to separate itself into a system side which is connected to the low voltage digital system, and an output side which is connected to the external load or system under control. With the galvanic isolation device, the system side and the output side are electrically and galvanic isolated from each other, but the digital control signal from the low voltage digital system could still be transmitted to the output side to control external load or system by the galvanic isolation device.
A conventional galvanic isolated digital output circuit, as shown in FIG. 1a, uses a photo coupler as the galvanic isolation device. Based on the ON/OFF digital control signals from the digital system output, a driving circuit 12 at the system side drives a light emitting device (i.e., the light emitting diode 11) of the photo coupler 10 to emitted light signals (represented by the arrow heads). A light sensing device (i.e., the photo transistor 13) at the output side of the photo coupler 10 detects the light signals and produces electrical signals corresponding to the ON/OFF digital control signal to drive the control circuit 14, which in turn controls the external load or system (i.e., the load 16).
The conventional galvanic isolated digital output circuit has a number of disadvantages. For example, the output side requires the configuration of another galvanic isolated power source 17, in addition to the power source 15 at the system side. Besides, the control circuit at the output side could only function either as a current source or as a current sink to the load, meaning a less flexible applicability in real life.
The galvanic isolation device is, as a matter of fact, a kind of energy conversion device. In addition to the foregoing photovoltaic devices (i.e., photo couplers), other possible means include thermoelectric devices which convert between thermal energy and electrical energy, piezoelectric devices which convert between vibration and electrical energy, and electromagnetic device which convert magnetic energy and electrical energy. One of the most common galvanic electromagnetic conversion devices is the transformer. For example, U.S. Pat. Nos. 4,052,623, 5,138,515, and 5,304,863 all disclose similar designs using a two-winding (one at the primary side and one at the secondary side) transformer to separate the circuit connected to the secondary winding for driving a power switch, and the control signal applied to the primary winding for turning on or off the power switch. Due to the electromagnetic conversion capability of the transformer, the power switch ON/OFF energy was delivered from the primary side and there is no need for the configuration of another galvanic isolated power source at the output side. The reference diagram of U.S. Pat. No. 4,052,623 is included as FIG. 1b as an example, whose details are omitted for simplicity sake.
The foregoing conventional digital output circuits have another major flaw which limits the control signal conversion in one direction from the system side to the output side only. Due to the galvanic isolation between the system side and the output-side, the digital system cannot obtain the output status of the output side, such as whether the external load or system under control responds correctly to the control signals. To achieve such a goal, a feedback circuit comprising another photo coupler, transformer, or similar galvanic isolation device is required, so that the electrical output status of the output side such as its voltage, current, frequency, and whether it is in failure conditions like open-circuited, short-circuited, or overloaded, could be fed back to the system side for the digital system's monitoring. Further more, the digital system would also require the configuration of additional input ports for connecting to the feedback circuit. All of the above increase the cost and wiring difficulty of the digital output circuit and the digital system.